I have used Gumtree and don't want to use www.yell.com as you end up with nothing but sales calls.
Some primary schools allow people to leave a pile of leaflets advertising tutoring in their main entrance, most schools just like to see a copy of a CRB check you've had done & are then happy for you to leave a pile of leaflets there, for interested children/adults to pick up.
You could try speaking to people who run things like church events, youth clubs etc and ask if you can stick up a couple of posters near the doorway, or again, leave a pile of leaflets there.
In some supermarkets people advertise tutoring on a 'customer notices' board. Not sure if it's free or not, but I imagine it wouldn't be too expensive.